Abstract

Fusarium wilt of banana is a fast spreading and persistent disease in all banana plantations throughout the world, including the southern provinces of China like Hainan, Guangdong and Fujian and its control has been extremely challenging. In this study, two strains of Streptomyces:BWL58and BWL15-4with great biological control potentials, were isolated from the soil of tropical virgin rain forests in Hainan. The research work involved the identification on these two strains, optimization of the solid-state fermentation conditions, elucidation of their antagonistic mechanisms and the application and evaluation of technology to control the disease in the field. The results are presented as follows:1. Based on the morphology, physiology, biochemistry and16S rDNA sequence analysis, BWL58and BWL15-4were identified respectively, as Streptomyces sahygroscopicus and Streptomyces albospinus.2. The optimization of solid-state fermentation media for the isolates was performed by determining the effects of pH, amount of inoculum, and temperature on the mycelium growth and spore production of these two strains. The carrier of the bio-control agents was based on the yield of the spores. Thus, the optimal conditions for the solid-state fermentation of BWL58included a medium composed of water, soy flour, wheat bran, sandy soil and rice at a ratio of150:5:20:30:100by weight, with initial pH at7.0, inoculum amounting to2%and incubation temperature at28℃. As for BWL15-4, the medium included water, soy flour, wheat bran, sandy soil and rice, with a ratio of130:10:20:20:100by weight, initial pH of6.3, inoculum set at3%, and incubation temperature at29.5℃. Under these conditions, BWL58and BWL15-4produced large amount of spores, up to1013cfu/g and1012cfu/g, respectively.3. In dual cultures with FOC on agar plates,both strains showed significant inhibition zones around FOC. Under the microscope, the bronzing of the edge of fungal hyphae could be seen. After the cell wall was lysed releasing the internal contents, the hyphae were twisted together. However, the protoplasts obtained from ruptured cells of both strains showed no suppression to FOC. In contrast, the secondary metabolites produced in broth cultures stopped the growth of FOC on agar plates, thus confirming that they were responsible for the antagonistism against FOC.4. The non-cultivated loess proved to be the best carrier of the bio-control agents. After3months, the amount of the spores remained high (109cfu/g) when the ratio of non-cultivated loess and microorganism was2:1in the mixture. When the sugarcane slag powder or sugarcane leaf powder was used as the carrier, the amount of the spores still maintained a level of at least108cfu/g after6months of storage.5. The field tests using carriers with these two antagonistic strains as the bio-control agents in banana only20%in nurseries with serious disease incidence, and it was determined that the most cost-effective organic fertilizer dosage should be kept at about1%.6. The results of field experiments base on our study condition,it showed that using a combination of resistant varieties and the bio-control agents discovered in this study effectively controlled the banana wilt disease with the incidence rate reduced below to10%.